Field of the Invention
The invention relates to an amplifier circuit containing at least one output stage transistor, a circuit for compensating for quiescent current drifts and optionally components for driving the output stage transistor(s).
Amplifier circuits for radiofrequency technology generally contain output stage transistors, such as e.g. GaAs-MESFETs. By way of example, the operating point of the output stage of a mobile telephone is adjusted to a region of the gain characteristic which lies in the lower third of the drain-source current I.sub.DS, typically at approximately I.sub.DS =0.1.times.I.sub.DSS (class AB operation).
Suitable amplifier circuits with high efficiency generally contain, on one chip, a plurality of MESFETs having a multiplicity of elongate doping regions. During fabrication, slight production-dictated fluctuations (e.g. due to varying doping concentration) may occur from chip to chip or from wafer to wafer, the fluctuations resulting in gain characteristics that are not exactly reproducible. Therefore, it is necessary to subsequently adjust the quiescent current for each individual chip. This is done by adjusting the so-called "pinch-off" voltage v.sub.p in a range of from e.g. -4 V to -2.5 V, with the result that I.sub.DS lies within a predetermined range of e.g. I.sub.DS =10% I.sub.DSS.+-.10%. An additional difficulty resides in the fact that the limit value for the quiescent current must be adhered to in the entire temperature range of from -30.degree. C. to 80.degree. C., for example. Complicated optimization of the quiescent current by subsequent calibration of the finished circuit is therefore customary. However, corresponding calibration is cost-intensive and complicated.
Active operating point adjustment for power amplifiers is likewise disclosed in Published, British Patent Application GB 2 307 155 A, Published, European Patent Application 0 529 421 A, an article by M. T. Schmidl et al., titled "Low Overhead, Low Complexity Burst Synchronization for OFDM", 1996 IEEE, Bd. 3, Pgs. 1301-1306, and an article by B. Stantchev et al., titled "Burst Synchronization for OFDM-Bade Cellular Systems With Separate Signaling Channel", VTC '98. 48.sup.th IEEE, Pgs 758-762, vol. 2.